Physics

Undersampling

Infrared

Chip

Laser

Optics

Astronomy

Frequency comb

Materials science

Optical frequency comb

Spectroscopy

Microwave

Computer science

Spectral line

Quantum mechanics

Comb generator

Optoelectronics

Telecommunications

Optica

With their compact size and semiconductor-chip-based operation, frequency microcombs can be an invaluable light source for gas spectrcoscopy. However, the generation of mid-infrared (mid-IR) frequency combs with gigahertz line spacing as required to resolve many gas spectra represents a significant challenge for these devices. Here, a technique referred to as interleaved difference-frequency generation (iDFG) is introduced that densifies the spectral line spacing upon conversion of near-IR comb light into the mid-IR light. A soliton microcomb is used as both a comb light source and microwave oscillator in a demonstration, and the spectrum of methane is measured to illustrate how the resulting mid-IR comb avoids spectral undersampling. Beyond demonstration of the iDFG technique, this work represents an important feasibility step towards more compact and potentially chip-based mid-IR gas spectroscopy modules.

Interleaved difference-frequency generation for microcomb spectral densification in the mid-infrared